As a system for recording digital data in an optical recording medium (optical disk), a system which implements a uniform recording density over the recording medium by holding a linear velocity constant, such as a CD (Compact Disc (registered trademark)) or a DVD (Digital Versatile Disk), is mostly used. In recent years, attention has been focused not only on a read-only optical disk but also on a recordable DVD-RAM (DVD-Random Access Memory), a once-writable DVD-Recordable (hereinafter referred to as DVD-R), and a rewritable DVD-Rewritable (hereinafter referred to as DVD-RW). Because the DVD-RAM disk has a characteristic feature of enabling recording/reproduction by random access, it is appropriate as an information recording medium.
The DVD-RAM disk has a plurality of unit blocks (sectors) each having headers in which address information and the like are written as embossed pits and a data portion in which digital data is actually recorded. In the headers and the data portion, a pull-in pattern (hereinafter referred to as a VFO pattern) configured to have a single frequency component is present. In a circuit for processing a reproduced signal, phase pull-in is performed on a per sector basis.
A phase locked loop (PLL) is known for performing the process, which reproduces a clock by switchingly using a phase error in an area with the VFO pattern and a phase error in an area where normal data is recorded and synchronizes the reproduced clock with the reproduced signal read from the DVD-RAM disk (see, e.g., Patent Document 1).
As a recording/reproduction speed increases, cost and power consumption increase with an increase in circuit scale. As one of methods which suppress the increasing cost and power consumption, there is known a method which sets the frequency of the reproduced clock to one-half of a channel bit frequency and performs sampling in synchronization with the reproduced clock in an analog to digital converter (see, e.g., Patent Document 2).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-200467    Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-36612